The transmitter and receiver of a transceiver generally operate at a different respective frequency. An antenna duplexer is often used to couple the transmitter and receiver to a common antenna. In essence an antenna duplexer comprises two different filters in the receive and transmit circuits respectively. The filter in the transmit circuit is generally designed to attenuate frequencies in a stopband including the operating frequency of the receiver, while the filter in the receiver circuit attenuates frequencies in a stopband including the operating frequency of the transmitter.
By way of example U.S. Pat. No. 4,462,098 discloses an antenna duplexer comprising receive and transmit filters each comprising a cascaded arrangement of discrete single pole ceramic bandstop/bandpass filters intercoupled by quarter-wave transmission lines. U.S. Pat. No. 4,431,977 discloses an antenna duplexer in which the receive and transmit filters are formed from a monolithic ceramic block comprising several integral resonators. The receive and transmit filters may be formed from separate blocks or they may be integrated into a single block. An older patent, U.S. Pat. No. 3,728,731 discloses an antenna duplexer in which the receiver and transmitter filters comprise an array of helical resonator filters mounted in a common housing. It is well-known that the electrical characteristics of a filter comprising a cascaded arrangement of individual resonators can be varied by changing the number of resonators and/or by altering the electrical characteristics of the individual resonators (or poles) of the filter and/or by altering the nature of the coupling between adjacent resonators.
In cellular radio telephone systems the separation between adjacent edges of the receive and transmit bandwiths may be relatively close. For example in the ETACS cellular system in the United Kingdom the transmission bandwith is 872-905 MHz and the receiver bandwith is 917-950 MHz. In this case the separation between adjacent edges of the receive and transmit bands is 12 MHz.
The problem which is encountered in cellular radio is to use a duplex filter which in the receive circuit attenuates the transmitter frequency without unduly attenuating the desired receive signal, and likewise in the transmit circuit attenuates the receiver frequency without unduly attenuating the transmit signal. This filtering problem becomes more severe when the transmitter is operating towards the top end of the transmit band, i.e. at a frequency close to the receive band, because the receive filter must still be able to discriminate and pass the desired receive frequency but attenuate the undesired frequency to a significant extent. In order to achieve higher filtering performance the duplex filter is generally provided with more poles and a more complex physical and electrical configuration. Consequently, duplex filters with high filtering performance tend to be both large in size and costly to manufacture. The high cost is an obvious disadvantage. The large size means that high performance filters are not compatible with portable transceiver apparatus where miniaturization is desirable.
A further disadvantage of high performance duplex filters is the associated insertion loss. That is to say, the larger the amount of filtering of the undesired signal, the higher is the loss to the desired signal.
In duplex radio transceivers it is widely recognized in the art that the most difficult filtering requirement is that of removing the relatively large transmitter power from the small signal circuits in the receiver. A greater level of transmitter power can be removed by increasing the receive filter performance. However, the larger the amount of filtering, then the higher is the loss to the desired signal, as mentioned above. In order to maintain usable receiver sensitivity there is a finite limit to the amount of loss that can be tolerated in the receiver circuit before amplification. In prior art transceivers it has been necessary to strike a delicate comprise in the receiver circuit between the requirement to reject transmitter frequencies and an acceptable level of insertion loss.
U.S. Pat. No. 3,656,162 discloses a diplexer for radio communication in which the antenna duplex filter comprises a passband filter in the receiver circuit, and in the transmit circuit a low pass combined with a notch filter tuned to reject signals at the receiver frequency. A drawback of this circuit is that the inclusion of a notch filter in the transmit circuit necessarily increases the insertion loss in the transmit circuit.